Physical inactivity had the greatest impact on a woman's lifetime risk for heart disease after age 30, according to an Australian study.

Action Points

Note that this Australian population-based study demonstrated that physical inactivity was a powerful risk factor for cardiovascular disease among Women.

Be aware that population attributable risks changed throughout the lifespan, with smoking being the greatest risk among women younger than 30.

Physical inactivity had the greatest impact on a woman's lifetime risk for heart disease after age 30, according to an Australian study.

Looking at four cardiovascular disease risk factors -- smoking, high body mass index (BMI), high blood pressure (BP), and physical inactivity -- among 32,154 Australian women, Wendy J. Brown, PhD, from the University of Queensland in St. Lucia, and colleagues found that, from age 30 until the late 80s, low physical activity was responsible for higher levels of population risk than any of the other risk factors.

In addition, when prevalence and relative risk data were combined to estimate population attributable risk (PAR), smoking was found to be the most important contributor to heart disease in adult women under the age of 30 -- a PAR of 59% compared with 15.2% to 29.2% for women between ages 47 to 64 and 5.3% for women 73 and older, they reported in the British Journal of Sports Medicine.

The results highlight the fact that population attributable risks for heart disease appear to change throughout women's lives, the researchers concluded.

"They show that continuing efforts to reduce smoking rates in young adult women are warranted, especially during the 20s," Brown's group wrote. "However, from about age 30, the PAR for inactivity outweighs that of the other leading risk factors, including high BMI, which is currently receiving much more attention in Australia than the 'Cinderella' risk factor -- physical inactivity."

Age-Related Risk

While it's known that BMI, smoking, high blood pressure, and lack of physical activity are all leading risk factors for heart disease, a better understanding of the varying contribution of each of these risk factors throughout adult life would help identify the population subgroups most in need of policy action, the researchers wrote.

Brown and colleagues analyzed data from the Australian Longitudinal Study on Women's Health, which is a prospective study of women recruited from the national Medicare health insurance database. Women in the study were born in the following time periods: 1921-1926, 1946-1951, and 1973-1978.

All participants completed mailed surveys in 1996, then each group was surveyed once every 3 years, on a rolling basis, beginning with the middle-age cohort in 1998.

Response rates for the various surveys ranged from between 72% and 99% of the numbers completing each preceding survey, and overall, from 42% in the older cohort (death and illness contributed to lower response) to 86% in the mid-age cohort, between the first and last surveys.

In an effort to quantify the importance of these leading risk factors on heart disease risk among women throughout their adult lives, the researchers conducted a PAR analysis, which is an "epidemiological metric used to define the proportion of a disease in a defined population that would be eliminated if exposure to a specific risk factor was eliminated."

Estimation of individual and combined PARs involved three steps:

Extraction of relative risk (RR) estimates for each risk factor. RRs for BMI were based on high (>23 kg/m2) versus low (≤23 kg/m2) BMI, RRs for smoking were for current versus nonsmokers, and RRs for BP were for high versus low (>115 mm Hg mean BP versus <115 mean BP). For physical inactivity, RRs were extracted for three levels of activity: no activity, low (1 to <600 metabolic equivalent [MET]-min/week) and moderate (600 to <1600 MET-min/week) relative to high (≥1600 MET-min/week).

Estimation of prevalence estimates for each of the risk factors, from three different study cohorts over 12 years.

Calculation of PARs. For high BMI, smoking, and high BP, PAR estimates were derived using the RR and prevalence estimates for each survey/age group. For physical inactivity, separate PAR estimates were produced for each of the none and low PA categories, which were then combined to provide a single overall estimate for each age group/survey. The "indicative range" of the PAR estimates was calculated using the low and high confidence interval values for each RR/prevalence pair, to provide an indication of the dispersion of PARs for each age group.

Age-Related Changes

For all four risk factors, RRs declined with age. The highest RRs were for smoking in the younger women and the lowest were for high BMI and high BP in the older women.

Although some young women quit and started again, the overall prevalence of smoking declined from 28% (95% CI 27.1-28.9) in 2000 to 11% (95% CI 10.6-12.0) in 2012. In contrast, the prevalence of most other risk factors increased over time.

Between the ages of 22 to 27 and 34 to 39, the proportion with high BMI increased from 46.1% (95% CI 45.1-47.2) to 65.6% (95% CI 64.5-66.7) and the proportion reporting no/low physical inactivity increased. The prevalence of high BP increased slightly, but remained below 5% at all surveys.

The prevalence of smoking among middle-age women almost halved, from 17% at age 47 to 52 to 9% at age 59 to 64. In contrast with the younger cohort, the prevalence of no activity remained constant (18%), but the prevalence of low activity decreased, from 35% to 28% over 12 years.

The highest prevalence of any risk factor was seen in this cohort for high BMI, which increased from 71% to 79% over time, with increases also in the prevalence of high BP from 19% to 31%.

The prevalence of smoking was just 5% in women between ages 73 to 78, and smoking was not measured again after this age. The prevalence of inactivity (no/low) increased markedly from 65% at ages 73 to 78 to 81% at age 85 to 90. Prevalence of high BMI was lower in the older cohort than in the mid-age cohort, declining from 68% at ages 73 to 79 to 62% at ages 85 to 90.

The PAR analysis revealed that the highest attributable fraction for risk of heart disease was for smoking in the younger cohort (ages 22 to 27 and 25 to 30). The PAR for smoking decreased markedly after this from 59% (ages 22 to 27) to 5% (ages 70 to 75).

In the younger cohort, the PAR for no physical inactivity remained constant throughout life, while the estimate for low activity was lower in the older than in the young and mid-age groups. When these estimates were combined, overall PAR was around 48% in the young cohort, 33% in the mid-age cohort, and 24% in the older cohort.

At baseline, PAR for high BMI was similar in the younger and older groups, and a downward trend in the mid-age cohort was seen. In the older cohort the PAR for high BMI declined with age from 15% (ages 73 to 78) to 11% (ages 85 to 90).

The PARs for hypertension were very low in the younger cohort (from 2% to 3% at all surveys) increasing to 7% to 11% in the mid-age and older cohorts, respectively.

Limitations and Implications

The study had some limitations including participant attrition over time that may have resulted in biases towards inclusion of healthy women. There also was a lack of data needed to adjust estimates for synergistic interactions, and the use of minimal risk criterion for physical inactivity and BMI.

Another limitation was the reliance on self-reports of hypertension and no objective measurements of blood pressure. As a result, the PARs for high blood pressure were likely to be "significantly underestimated," especially in older women.

The study findings highlight the importance of emphasizing regular exercise for reducing cardiovascular disease risk, especially in young adulthood and middle age, the researchers said.

"Our data suggest that national programs for the promotion and maintenance of physical activity, across the adult lifespan, but especially in young adulthood, deserve to be a much higher public health priority for women than they are now," they wrote.

They estimated that "if every woman between the ages of 30 and 90 were able to reach the recommended weekly exercise quota -- 150 minutes of at least moderate intensity physical activity -- then the lives of more than 2,000 middle-age and older women could be saved each year in Australia alone."

Accessibility Statement

At MedPage Today, we are committed to ensuring that individuals with disabilities can access all of the content offered by MedPage Today through our website and other properties. If you are having trouble accessing www.medpagetoday.com, MedPageToday's mobile apps, please email legal@ziffdavis.com for assistance. Please put "ADA Inquiry" in the subject line of your email.